1. Field of the Invention
This invention relates to an optical wavelength conversion system which converts the wavelength of a laser beam from a semiconductor laser by use of an optical waveguide type optical wavelength conversion element which converts a fundamental wave to a second harmonic, and more particularly to such an optical wavelength conversion system which converts the wavelength of a laser beam from a semiconductor laser by use of an optical wavelength conversion element comprising an optical waveguide formed on a ferroelectric crystal substrate and periodic domain reversals formed in the optical waveguide.
2. Description of the Related Art
There has been proposed by Bleombergen and et al. a method of converting a fundamental wave to a second harmonic by use of an optical wavelength conversion element formed with a region where the spontaneous polarization (domain) of a ferroelectric material having a nonlinear optical effect is periodically reversed. (See Phys. Rev., vol. 127, No. 6, 1918 (1962)) In this method, by setting pitches .LAMBDA. of the domain reversals to an integer multiple of the coherence length .LAMBDA.c given by formula .LAMBDA.c=2.PI./{.beta.(2.omega.)-2.beta.(.omega.)}, wherein .beta.(2.omega.) represents the propagation constant of the second harmonic and .beta.(.omega.) represents the propagation constant of the fundamental wave, phase matching (artificial phase matching) between the fundamental wave and the second harmonic can be obtained.
Further there have been made attempts to efficiently obtain the phase matching in an optical wavelength conversion element, which has an optical waveguide formed of a nonlinear optical material and converts the wavelength of a fundamental wave guided along the optical waveguide, by forming the domain reversals described above. See, for instance, Japanese Unexamined Patent Publication No. 7(1995)-152055.
Such optical waveguide type optical wavelength conversion elements with periodic domain reversals have been often used for wavelength conversion of a laser beam emitted from a semiconductor laser. In this case, unless the oscillation wavelength of the semiconductor laser is a wavelength which is matched in phase with the pitches .LAMBDA. of the domain reversals, the wavelength conversion efficiency significantly deteriorates and it becomes difficult to obtain a short wavelength light source which can be practically used.
Thus, it has been proposed as disclosed, for instance, in the above identified Japanese Unexamined Patent Publication No. 7(1995)-152055 to tune and lock the oscillation wavelength of the semiconductor laser at a desired value by providing a narrow band pass filter in the optical path of the laser beam between the semiconductor laser and the optical wavelength conversion element.
An example such an arrangement is shown in FIG. 4. In FIG. 4, reference numerals 1 to 4 respectively denote a semiconductor laser 1 which generates a laser beam 2 as a fundamental wave, an incident optical system 3, and an optical waveguide type optical wavelength conversion element 4 having a channel optical waveguide 4a and periodic domain reversals 4b. The incident optical system 3 comprises a collimator lens 5 which collimates the laser beam 2 emitted from the semiconductor laser 1 as divergent light, a condenser lens 6 which converges the collimated laser beam 2, a .lambda./2 plate 7 for polarization control disposed between the lenses 5 and 6, and a narrow band pass filter 8, which may be, for instance, a dielectric multi-layered film filter.
Such a narrow band pass filter 8 has spectral transmission properties which are substantially as shown in FIG. 5. Further the peak transmission wavelength .lambda.0 in the properties generally depends upon the light incident angle .theta. to the band pass filter 8 as shown in FIG. 6. The laser beam 2 passing through the band pass filter 8 with such properties is converged on the light incident end face of the optical waveguide 4a by the condenser lens 6 and is divided into a component which enters the optical waveguide 4a in a TM mode and a component which is reflected at the light incident end face of the optical waveguide 4a.
The laser beam 2 input into the waveguide 4a is converted to its second harmonic 9 after passing through the periodic domain reversals 4b. On the other hand, the laser beam 2 reflected at the light incident end face of the optical waveguide 4a is fed back to the semiconductor laser 1 retracing its optical path to the waveguide 4a, and resonates between the light incident end face and the rear end face of the semiconductor laser 1, whereby the semiconductor laser 1 comes to oscillate at a wavelength of .lambda.0.
Since the peak transmission wavelength .lambda.0 of the band pass filter 8 depends upon the light incident angle .theta. to the band pass filter 8 as shown in FIG. 6, the oscillating wavelength of the semiconductor laser 1 can be tuned and locked at a value which is matched in phase with the pitches .LAMBDA. of the domain reversals by rotating the band pass filter 8 as shown by arrow A in FIG. 4.
However, in the conventional system, even if the laser beam 2 has correctly impinged upon the light incident end face of the waveguide 4a until tuning as shown in FIG. 7A, the optical path of the laser beam 2 can be inclined as shown by the dashed line in FIG. 7B when the band pass filter 8 is rotated for tuning. This is due to the fact that the band pass filter 8 is not of a perfect parallel plate and that the laser beam 2 has not been perfectly collimated.
When the optical path of the laser beam 2 is inclined, the position on the incident end face of the waveguide 4a in which the laser beam 2 is converged is shifted. Though being small, the shift results in poor optical coupling efficiency of the laser beam to the optical waveguide and poor output of the wavelength-converted wave since the waveguide is generally as small as 2 to 3 .mu.m in diameter. At the worst, the amount of light entering the waveguide can become extremely small to such an extent that tuning the oscillation wavelength becomes impossible.
Further in the conventional system, there has been a problem that since production of a band pass filter which is high in transmittance is difficult, the amount of light entering the waveguide is apt to be small and accordingly it is difficult to obtain a high output wavelength-converted wave.
Specifically, in order to cause a semiconductor laser to oscillate in a single longitudinal mode, the transmission wavelength half-amplitude level of the band pass filter generally should be not larger that 0.5 nm. Very high film forming technique is required to form a band pass filter with such a transmission wavelength half-amplitude level and a high transmittance by multilayered film forming technic. For example, when trying to produce a band pass filter with a transmission wavelength half-amplitude level of 0.5 nm and a transmittance of not lower than 80% by conventional multilayered film forming technique, yield in the film forming process becomes very low, which results in a very expensive band pass filter. According to the conventional ordinary film forming technic, the transmittance of a band pass filter whose transmission wavelength half-amplitude level is 0.5 nm can be up to about 30% and accordingly the output of the wavelength-converted wave is very poor.